A heat recovery ventilator (HRV) is a mechanical device that incorporates a heat exchanger with a ventilation system for providing controlled ventilation into a building. The heat exchanger heats or cools the incoming fresh air using the exhaust air. Devices that also exchange moisture between the two air streams are generally referred to as Energy Recovery Ventilators (ERV), sometimes also referred to as Enthalpy Recovery Ventilators or enthalpy exchangers.
In order for buildings to have good indoor air quality they require an exchange of the stale indoor air with fresh outdoor air. An ERV can be used for this purpose, and incorporates a method to remove excess humidity from, or add humidity to, the ventilating air that is being brought into a building. In addition to improving indoor air quality in buildings, installation of an ERV will result in energy savings. For example, in hot and humid climates, useable energy is wasted when the cooled air from the building is exhausted. In an ERV the exhaust air can be used to cool the warmer air being brought in from the outside, reducing the energy consumption load on the air conditioner and the energy associated with air conditioning. With proper design, the size of the air conditioner can be reduced. If buildings tend to be too humid, ERVs can lower humidity levels, reducing the likelihood of mould, bacteria, viruses, and fungi which cause sickness, absenteeism and lost productivity. On the other hand, in cold dry climates, energy is wasted when warm air from the building is exhausted, plus there can be an additional issue of the incoming air stream being too dry. As well as transferring heat from the exhaust air to the incoming air, ERVs can be used to recycle water vapour from the exhaust stream, raising humidity levels, thereby reducing skin irritation, dryness, and respiratory symptoms caused by dry air.
A key component in the ERV system which transfers the heat and humidity between the air streams is called the ERV core. The two most common types of ERV are those based on planar membrane plate-type devices and those based on rotating enthalpy wheel devices. Planar plate-type ERV cores comprise layers of water permeable membrane. The two air streams are directed through alternate layers, or on opposite sides, of the ERV core, and heat and humidity is transferred via the membrane. Enthalpy wheel ERVs (also known as energy wheels) typically have a cylindrical or disc-shaped honeycomb core that is coated with desiccant. A motor rotates the cylinder, transferring the heat and humidity between the intake and exhaust air streams. ERV systems typically also comprise an enclosure, pumps or fans to move the air streams, ducting, as well as filters, control electronics and other components.
Since the air is being exhausted primarily to remove stale and contaminated air from the building, preferably the exhaust stream should not be able to mix with the incoming stream on the opposite side of the membrane as the two streams pass through the ERV. However, in many cases there is crossover contamination (leakage between streams) due to leakage at seals or joints in the ERV and/or due to passage of the gases through the membrane material.
Preferably the membrane used in an ERV core is thin to allow adequate exchange of heat between the two streams, driven by the temperature gradient between the streams. The membrane is also water permeable to allow moisture to pass through the material, driven by the vapour pressure differential or water concentration gradient between the two streams. Thinner membranes will tend to have higher heat and moisture transport rates. Ideally the membrane is also impermeable to air, and contaminant gases, to prevent the mixing and crossover of the two streams through the membrane.
Membranes that have been used or suggested for ERV applications include cellulose films; cellulose fibre or glass fibre papers or porous polymer films that are coated or impregnated with a hydrophilic polymer or a hydrophilic polymer-desiccant mixture; thin film composites manufactured via interfacial polymerization; laminated membranes made from a blown film on a non-woven support layer; laminated membranes comprising an ionomer film bonded to a porous support; and sulphonated and carboxylated ionomer films. Other materials involve applying a water permeable coating to the microporous substrate. All of these materials have shortcomings however. For example, cellulose films are not mechanically and dimensionally stable in wet conditions, tend to be subject to freeze/thaw cracking, and are typically fairly thick (for example, greater than 10 micron) which leads to lower water permeance. In the presence of liquid water, water-soluble components tend to wash off papers or polymer films that are coated with hydrophilic polymers and/or polymer-desiccant mixtures. When a desiccant is added to the coating, this can necessitate high loadings of desiccant (>80%) and thick coating layers in order to block gas transport; this can reduce water transport. Cellulose films and coated papers also tend to be flammable and subject to microbial growth. In the case of thin film composites manufactured via interfacial polymerization, monomers are reacted on the surface of a porous polymeric substrate to make a chemically-bound water permeable coating, in order to reduce the problem of the coating washing off. Such membranes tend to be costly and their fabrication involves the use of organic solvents and other harsh chemicals. Also, the types of additives that can be incorporated are limited by the chemistry involved. Laminated membranes made by bonding a cast ionomer film to a porous support, or a blown film (for example, polyether-block amide (PEBA)) laminated to a nonwoven tend to delaminate because of the different dimensional properties (for example, swelling and thermal expansion) of the two layers and the difficulty in creating a strong bond between them. Also, the water transport performance of such laminated membranes tends to be limited because the ionomer or blown film has to be sufficiently thick (for example, greater 5 microns) so that it can be processed into a continuous, pinhole-free film and then handled in order to manufacture the laminate.
Desirable properties of a membrane for enthalpy exchangers, and other applications involving exchange of moisture and optionally heat between gas streams with little or no mixing of the gas streams through the membrane, generally include the following:                High water permeation (vapour and liquid);        High water absorption;        Low or zero air and contaminant gas permeation;        Non-flammable;        Resistance to microbial growth;        Favorable mechanical strength and properties when dry or when wet, so that the membrane is easy to handle, does not tear easily, preferably will accept and hold a pleat, and is stiff enough to withstand pressure differentials so the membrane does not deflect unduly;        Good dimensional stability in the presence of liquid water and washable, allowing cleaning for maintenance purposes without damaging or compromising the functionality of the ERV core;        Long lifetime under the required operating conditions, without detrimental leaching or loss of membrane components and without significant degradation in water vapour transport performance or increased contaminant crossover;        Tolerance to freeze-thaw cycles in the presence of liquid water condensation without significant deterioration in performance;        Low cost;        Formability, meaning the membrane can be formed into three-dimensional structures and will hold its formed shape.        
Often the above represent conflicting requirements. For example, materials which have low air permeability tend to also have low water permeability; polymer films provide excellent handling, but tend to be rather flammable; and specialty polymers and highly engineered thin film composites and similar materials tend to be very expensive.
Some of the most promising state-of-the-art membranes for these devices are porous desiccant-loaded polymer substrates coated with a thin layer of water permeable polymer, for example, as described in WO2010/132983 which is hereby incorporated by reference. The substrate provides structural rigidity to the membrane while the thin functional polymer layer provides water vapour transport selectivity. Utilizing such substrates allows the application of a thin water permeable polymer layer (for example, the thickness of the coating can be less than 5 microns, and is preferably less than 1 micron) which improves the water vapour permeation performance of these materials, although there is still some resistance to water transport through the coating layer. The thickness of the substrate is typically in the range of 50-200 microns, and testing indicates that more than 50% of water vapour transport resistance in such membranes comes from the substrate. This is related to the nature of the porous substrate, which tends to have tortuous pores and dead-ended pores, leading to increased resistance to water transport.
One of the key ways to increase enthalpy exchange efficiency in ERVs and other devices is by decreasing the water vapour transport resistance of the substrate material. At the same time it is important not to increase the thickness of the selective layer. A further way to increase performance is to improve the water permeance of the selective polymer layer. However, many higher permeability polymers are cost prohibitive for these applications. The substrate layer represents a large portion of the cost and transport resistance in current materials. If this layer can be eliminated, the membrane cost will be drastically decreased while the vapour transport performance will be increased. For example in one current generation membrane material for ERV applications, the cost of the microporous substrate layer is over 80% of the membrane cost. If the microporous substrate can be eliminated from the membrane, there may be an economic justification for using more costly, but higher permeability coating materials and additives.
Membranes which are formable are advantageous in the assembly and fabrication of membrane modules. Engineered composite membrane materials which can be formed into self-supporting three-dimensional structures, will allow increased performance and decreased cost in membrane-based devices.
The water vapour transport membranes described herein can provide high water permeance and high selectivity (low gas crossover) making them particularly suitable for ERV applications, and other applications involving exchange of moisture and optionally heat between gas streams. Furthermore, membranes which have similar permeation and selective properties which can also be formed into three-dimensional structures are demonstrated.